**Core Concept**
Hypertension in the postpartum period can be a sign of underlying chronic hypertension or preeclampsia. The American College of Cardiology (ACC) and the American Heart Association (AHA) define postpartum hypertension as elevated blood pressure persisting for more than 3 months after delivery.

**Why the Correct Answer is Right**
Preeclampsia is a condition characterized by new-onset hypertension and proteinuria after 20 weeks of gestation. In some cases, preeclampsia can persist postpartum, leading to chronic hypertension. The renin-angiotensin-aldosterone system (RAAS) plays a crucial role in blood pressure regulation, and its dysregulation can contribute to postpartum hypertension. **The sympathetic nervous system also plays a role in blood pressure regulation, particularly in the postpartum period.**

**Why Each Wrong Option is Incorrect**
**Option A:** While gestational hypertension can occur during pregnancy, it typically resolves postpartum, making this option incorrect for a condition persisting for > 3 months.
**Option B:** Preeclampsia is a specific condition that typically develops after 20 weeks of gestation; it's not a broad term that encompasses all postpartum hypertension.
**Option C:** Chronic kidney disease can cause hypertension, but it is not the primary cause of postpartum hypertension.

**Clinical Pearl / High-Yield Fact**
Postpartum hypertension can be a sign of underlying chronic hypertension or preeclampsia, which can increase the risk of cardiovascular disease and other complications.

**Correct Answer: B. Preeclampsia.**